In devices for providing ultraviolet (UV) wavelength radiation it is usual to generate the ultraviolet wavelength radiation by harmonic conversion of visible wavelength radiation in an optically nonlinear crystal. Typically, the visible wavelength radiation is generated by second-harmonic conversion of near-infrared (NIR) wavelength radiation, which is generated in a solid-state laser, such as an optically-pumped semiconductor (OPS) laser.
By way of example, in one common arrangement for generating continuous-wave UV radiation, the visible radiation is generated by intra-cavity second-harmonic conversion of fundamental radiation having a wavelength in the NIR region of the electromagnetic spectrum. The visible radiation is coupled into an impedance-matched resonant-enhancement cavity for the visible radiation. An optically nonlinear crystal within the resonant-enhancement cavity converts the visible radiation to UV radiation. Using an OPS laser for the intra-cavity generation of the visible radiation, it is possible to generate UV radiation having a wavelength of 266 nanometers (nm) or less.
Typically, the UV radiation is directed out of the resonant cavity by an optically-coated thin-film dichroic filter. This dichroic filter may serve as one of the resonator mirrors, reflecting the visible radiation and transmitting the UV radiation. Alternatively, the dichroic filter may also be an intra-cavity element, transmitting the visible radiation and reflecting the UV radiation out of the resonator.
A particular limitation of the laser arrangements described above is damage to thin-film optical coatings caused by the UV radiation. This limitation becomes more problematic for shorter-wavelength UV radiation. Further, even a not-yet-damaged coating will typically have losses due to manufacturing variances in layer-thickness or due to absorption by the materials of the coating. Such losses reduce the efficiency of resonators having intra-cavity harmonic generation and ultimately the useful lifetime of these resonators. There is a need for a device to separate UV radiation from visible radiation that does not require any thin-film optical coatings.